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Operating Systems
Operating Systems An operating system is a collection of integrated , interrelated and interdependent groups of programs , which are designed to perform various internal operations of Computer efficiently and make it easier for people to use the computer . Tasks of Operating System : 1) '''Recognizing input from input devices '''2) '''Sending Output to Output devices 3) keeping track of files and directories on secondary storage devices '''4) Controlling Peripheral Devices Classification of Operating Systems : # Multi-User : Allows two or more users to run programs at the same time . # Multi-Processing : Allows a single program to run on more than one CPU . # Multi-Tasking : Allows two or more processes to run simultaneously . # Multi-Threading : Allows different parts of a program to run simultaneously . # Real Time :''' Responds to input instantly . (e.g. DOS , UNIX are not real time) Information Management Information Management (IM) refers to a set of services used for storing , retrieving , modifying or removing the information on various devices . IM consists of two main modules : '''1) File system .... 2) Device Driver (DD) or Device Management (DM) Block : A block is a logical unit of data that the operating system defines for its convenience . Files : A file in a computer refers to a set of data and/or instructions . Every file generally has these properties : name , extension , size , type , etc. Directories (Folders) : A directory is a logical structure or Object which is linked to sub-directories and files that behave as if they are stored within it . VTOC or Device Directory : VTOC stands for Volume Table of Contents . For every file , the OS maintains a VTOC , which stores file information such as File Name , Extension , Size , etc. System Calls : '''The system calls under IM are : create a file , open a file , close a file , Write data form buffer to head , create link , create a directory , Explore a directory , read data from file to buffer , move file pointer , change working directory . File Operations '''File Access Methods Files store Data . When a file is being used , its data is to be accessed : # Sequential Access : Records are sequentially stored one after the other . The drawback is that the application has to go through the records sequentially before finding the required record . # Direct Access : In this method , any block can be read or written at random without having to search sequentially . Allocation Methods These are methods of storing data and allocating memory to data . # Contiguous Allocation : In this method , every file is allocated consecutive blocks on storage device . The drawback with this method is the relocation of free space to new files and limit on maximum file size . If we want to edit a file , then it may require more space . Thus , contiguous allotment uses a technique called Dynamic Allocation . In dynamic allocation , disk space is perceived as a large array of blocks . The OS searches through the 'holes and determines which is the best hole to insert data . For this , the OS uses three methods : First Fit , Best Fit , Worst Fit . First Fit technique allocates the first big enough space . Best Fit technique allocates the smallest hole which is big enough to store data . Worst Fit technique allocates the largest hole to store data . # Linked or Chained Allocation : In this method , the file may be scattered in different locations or blocks . This directory contains a pointer to first and last block in the file . In this method , direct access is not possible . # Indexed Allocation : All pointers are brought together into one block called as index block . Each file has its own index block . A file allocation table is maintained by an OS on a floppy or hard disk that provides a map of clusters that a file has been stored in . File Systems File Systems : '''The collection of related information is called as a file system . For convenient use , the OS provides systematic list of file directories and systems . There are two types of file systems : disk-based and tape-based . '''Disk based Systems : '''Each disk is divided into tracks and each track is dividd into sectors . A sector is the smallest unit of unformation which can be read from and written to the disk . '''Sector varies from 32 bytes to 4096 bytes . A track contains 4 to 32 sectors per track and from 75 to 500 tracks per disk . The time requird for read/write heads to move to correct tracks is called as Sek Time '. The time required to activate Read/Write Heads for appropriate surface is called as '''transmission time '. The time rquired for requested sector on track to rotate blow the had is called as '''Latency time or rotational delay . Tape-based Systems : '''They are simple but inefficient . '''Device Drivers Device Drivers are software programs required for each device . Each device requires different drivers based on its functions . A device driver knows how should the buffers , flags , register control and status bits should be used for a device . Some device drivers are useful for data conversions . Process Management In Multiuser operating systms many users might execute same or different programs at the same time . But , the computer systems have only one CPU and it can execute only one instruction at a time . Thus , the instructions need to be scheduled and tracked . This is called as Process Management . The system calls under process management are : ready a process , resume a process , suspend a process , block a process , change the priority of a process , terminate a process , delay a process , generate a process , fork a process . Memory Management When a program is being executed , the OS allocates memory space to it . But the OS should know how much free space is available and how much memory is needed for the task . For this , the OS keeps a list of free memory locations . Before starting the execution of the program , the OS checks the list and allocates space to the program . This is called as Memory Management . The system calls under memory management are : to allocate a chunk of memory to a process , to free chunk of memory from a process . Graphical User Interface (GUI) In Graphical User Interface , the commands are replaced by graphical representations , so that the user need not know the keywords and can use the program easily . Security Agents Worms and Virus Worms : '''A computr worm is a computer program , which spreads to other computers over the network . While spreading , it consumes network resources to a large extent . A computer worm can potentially bring the entire network to a standstill . A Computer Worm generally does not harm other programs or data . It consumes transmission capacity and disk storage . It may deny access to legitimate users . The worm spreads over a network . It gets address of other computers on the network through mailing list and sends a copy of itself to other computers . A worm can be prevented by strong security and various check points on the communication system . '''Virus : A computer Virus is a part of a program , which is written to infect other programs . It is not a complete program , but comes with installation of other programs . A virus can cause direct harm to the computer . It can corrupt code as well as data . The five types of viruses are : 1) Boot Sector Virus , 2) Memory Resident Virus , 3) File Specific Virus , 4) Command Processor Virus 5) General Purpose Virus . The well known mthods of infection by virus are : # Append : The viral code appends itself to unaffected programs . # Replace : The viral code replaces itself with original program code . # Insert : The viral code is insrted in the body of orinal code , so that the program misbehaves . # Delete : The viral code deletes some code from original program , # Redirect : The normal flow of a program is disrupted and the virus forces the user to be redirected to some other program or viral program . Detection of Virus can be done by cchecking the integrity of binary files . THe program maintains a check of all these files , a mismaatch in it indicates virus . Some programs continuously monitor the memory locations to check for virus . Removal of Virus is not easy because the bit pattern of virus may vary . If th bit pattern of virus is dtected then it can be removed . Prevention '''of Virus can be done by buying only legal copies of software from reliable sources . Illegal and free softwares have higher risk of containing virus . Frequent back-ups and running of monitoring programs also help in detection and removal of virus . Windows 98 Overview # Windows 98 is a single user multi tasking operating system developed by Microsoft . # Windows 98 is easier to use . Navigating around the computer is easier . # A file , directory or a program can be opened with a singl click . # Windows 98 allows you to use multiple monitors with the same computer . # New hardware can be easily installed without having to restart the computer . # With Windows 98 , you can use cameras and other digital imaging devices . # Windows 98 can be easily integrated with Internet . Web pages can be viewed in any window . E-mail can be linked to application called Microsoft Outlook . # Windows 98 supports DVD and digital audio , thus users can play high definition digital movies and audio on computer . Windows NT Overview # Windows NT is a multi user , multi tasking and multi threading operating system developed by Microsoft . # A user can get faster response even though multiple programs are running . # Windows NT supports virtual memory management system , # It is a 32 bit Operating system . # Windows NT provides New Technology File Systems , which implements fault toletance , security and has support for very large files . Linux Overview # Linux is a Free to Use operating system . It can be used , modified in any way . # It is a multiuser , operating system with a full set of unix compatible tools . # Linux has compatibility on a wide range of platforms . # It can run even on machines having 4 GB RAM . # It has standard interfaces for both the programmer and the user . # Linux supports a wide range of applications . # The file system in Linux obeys UNIX semantics . The components of LINUX are Kernel , System Libraries and System utilities . '''Kernel : '''Kernel maintains all important abstractions of the operating systems , such as processes and virtual memory . '''System Libraries : '''System libraries define a standard set of functions through which applications can interact with the kernel , and which implements much of the OS functionality . '''System Utilities : These are programs that performs individual , specialized management tasks . Some system utilities may be invoked just once to initialize and configure some aspects of systems . Test Yourself ! __INDEX__ Category:Computer Science